Lithium-titanium-oxide anodes for lithium batteries

ABSTRACT

A spinel-type structure with the general formula Li[Ti 1.67 Li 0.33−y M y ]O 4 , for 0&lt;y≦0.33, where M=Mg and/or Al. The structure is usefid as a negative electrode for a non-aqueous electrochemical cell and in a non-aqueous battery comprising an plurality of cells, electrically connected, each cell comprising a negative electrode, an electrolyte and a positive electrode, the negative electrode consisting of the spinel-type structure disclosed.

RELATED APPLICATION

This application, pursuant to 37 C.F.R. § 1.78(c), claims priority basedon provisional application serial no. 60/092,181 filed Jul. 9, 1998.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. W-31-109-ENG-38 between the U.S. Department of Energy (DOE)and The University of Chicago representing Argonne National Laboratory.

BACKGROUND OF THE INVENTION

Rechargeable lithium battery technology has become increasinglyimportant in recent years because it is providing new, lightweight, highenergy density batteries for powering applications in the rapidlygrowing electronics industry. These batteries are also of interestbecause of their possible application in electric vehicles and hybridelectric vehicles. State-of-the-art rechargeable lithium batteries areknown as “lithium-ion” batteries because during charge and discharge,lithium ions are shuttled between two host electrode structures with aconcomitant reduction and oxidation of the host electrodes. The bestknown lithium-ion cell is a 3.5 V Li_(x)C₆/Li_(1−x)CoO₂ cell, in whichlithium is extracted from a layered LiCoO₂ structure (positive electrodeor cathode) during charge and inserted into a carbonaceous structure(negative electrode or anode), typically graphite or a “hard” orpyrolyzed carbon. Lithiated carbons can approach and reach the potentialof metallic lithium at the top of the charge cycle. Therefore, thesenegative electrodes or anodes are highly reactive materials,particularly in the presence of a highly oxidizing Li_(1−x)CoO₂ positiveelectrode and a flammable organic electrolyte. There is, therefore, aconcern about the safety of charged lithium-ion cells; sophisticatedelectronic circuitry has to be incorporated into each cell to protectthem from overcharge and abuse. This invention addresses the need tofind alternative negative electrode materials to carbon.

The spinel, Li₄Ti₅O₁₂, is an attractive alternative negative electrodematerial to carbon. Three lithium ions can be inserted into thestructure according to the reaction:

3Li+Li₄Ti₅O₁₂→Li₇Ti₅O₁₂

This reaction occurs at approximately 1.5 V vs. metallic lithium,thereby providing a relatively safe electrode system compared to carbon.However, safety is gained at the expense of cell voltage and energydensity. A further limitation is that Li₄Ti₅O₁₂ provides a relativelylow theoretical capacity (175 mAh/g) compared to lithiated graphite(LiC₆, 372 mAh/g). Nevertheless, despite these limitations, cells withlithium-titanium-oxide negative electrodes can still be coupled to highvoltage (4 V) positive electrode materials, such as the layered oxidesLiCoO₂, LiNiO₂, LiCo_(1−x)Ni_(x)O₂, and spinel oxides, for example,LiMn₂O₄ to provide cells with an operating voltage of between 2.4 and2.2 V. It is anticipated that these cells will become increasinglyattractive from at safety standpoint, particularly as the voltagerequirement for powering semiconducting devices decreases in time. Forexample, a 2.4 V lithium-ion cell can be constructed by coupling twospinel electrodes:

Li_(4+x)Ti₅O₁₂+3 Li_(1−x/3)Mn₂O₄<- - ->Li₄Ti₅O₁₂+3 LiMn₂O₄

From a structural viewpoint, Li₄T₁₅O₁₂ is an example of an almost idealhost electrode for a lithium-ion cell. Lithium insertion into the cubicLi₄T₁₅O₁₂ spinel structure occurs with virtually no change in thelattice parameter (8.36 Å); the unit cell expands and contractsisotropically during lithium insertion and extraction, thereby providingan extremely stable electrode structure; it can undergo many hundreds ofcycles without structural disintegration. Moreover, lithium insertioncauses a first-order displacement of the tetrahedrally-coordinatedlithium ions in the Li₄Ti₅O₁₂ spinel structure into octahedral sites togenerate the ordered rock salt phase Li₇Ti₅O₁₂. The insertion (andextraction) of lithium is thus a two-phase reaction which provides aconstant voltage response (at approximately 1.5 V). Furthermore, thevoltage of a Li/Li_(4+x)Ti₅O,₁₂ cell changes abruptly at the end ofdischarge and charge. Thus, a Li_(4+x)Ti₅O₁₂ spinel electrode providesvery sharp end-of-charge and end-of-discharge indicators which is usefulfor controlling cell operation and preventing overcharge andoverdischarge.

A major disadvantage of a Li₄Ti₅O₁₂ spinel electrode is that all thetitanium ions in the structure are tetravalent; the material is thus aninsulator, with negligible electronic conductivity—it is white in color.Good insertion electrodes should have both good ionic conductivity toallow rapid lithium-ion diffusion within the host and good electronicconductivity to transfer electrons from the host structure to theexternal circuit during charge and discharge. To overcome poorelectronic conductivity, it is customary to add an electronic currentcollector, such as carbon, to metal oxide host electrodes. Thus,throughout the discharge and charge processes, the two-phaseLi_(4+x)Ti₅O₁₂ electrode will consist of an insulating Li₄Ti₅O₁₂ spinelphase (in which the titanium ions are all tetravalent) and amixed-valent, electronically-conducting rock salt phase Li₇Ti₅O₁₂, inwhich the mean oxidation state of the titanium ions is 3.4 (i.e., 60%Ti³⁺ and 40% Ti⁴⁺). Thus, when lithium is extracted from Li₇Ti₅O₁₂, theinsulating phase Li₄Ti₅O₁₂ will be formed at the surface of theelectrode particles. The insulating properties of Li₄Ti₅O₁₂ will inhibitelectronic conductivity at the surface of the particles, thus reducingthe rate of electron transfer and the capability of the cell to passcurrent.

SUMMARY OF THE INVENTION

This invention relates to new modified lithium-titanium-oxide materialswith a spinel-type structure with improved electronic conductivity. Thematerials are of particular interest as negative electrodes (anodes) forlithium cells and batteries, and, in particular, rechargeable lithiumbatteries. The invention therefore includes new materials, methods ofmaking the materials, the use of the materials as electrodes (negativeor positive) in lithium cells and batteries, and cells and batteriesemploying such electrodes.

It would thus be advantageous to prepare a lithium titanium oxide spinelthat is electronically conducting to enhance the performance of thestandard insulating Li₄Ti₅O₁₂ electrode. This invention relates to thepreparation of novel substituted Li₄Ti₅O₁₂ spinel materials andelectrodes, Li_(4−x)Ti₅M_(x)O₁₂(0<x≦1), or alternatively in spinelnotation Li[Ti_(1.67)Li_(0.33−y)M_(y)]O₄(0<y≦0.33), in which the lithiumions on the octahedral sites are partially substituted by M cations,where M is Mg²⁺ and/or Al³⁺, to reduce the oxidation state of thetitanium ions, thereby generating a mixed-valent Ti⁴⁺/Ti³⁺ couple withinthe spinel structure at all states of charge and discharge and enhancingthe electronic conductivity of the spinel electrode. The invention isextended to include the family of spinel compoundsLi[Ti_(1.67)Li_(0.33−y)M_(y-z)M′_(z)]O₄ that can be derived fromLi[Ti_(1.67)Li_(0.33−y)M_(y)]O₄ by partial substitution of the M cationsby M′ cations where 0<y≦0.33, z<y, M is Mg²⁺ and/or Al³⁺, and M′ is oneor more suitable monovalent, divalent, trivalent and tetravalent metalcations. The M′ metal cations are selected from the first row oftransition metals, preferably from Co⁺³, Co⁺², Ni⁺² and Ni⁺³. The metalcations M′, are most preferably selected from those ions that have ionicradii comparable to the Li⁺, Ti⁴⁺ and Ti³⁺ ions within the[Ti_(1.67)Li_(0.33−y)M_(y)]O₄ spinel framework, preferably within 0.15Å.

The principles of this invention will be described by particularreference to the lithium-titanium-oxide spinel systemLi_(4−x)Ti₅M_(x)O₁₂(0<x≦1), or alternatively in spinel notationLi[Ti_(1.67)Li_(0.33−y)M_(y)]O₄(0<y≦0.33), in which magnesium andaluminum are used as the substituting metal cation M. In general, theelectrochemical properties of the spinel electrodes have been evaluatedin cells against a lithium counter [reference] electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention consists of certain novel features and a combination ofparts hereinafter fully described, illustrated in the accompanyingdrawings, and particularly pointed out in the appended claims, it beingunderstood that various changes in the details may be made withoutdeparting from the spirit, or sacrificing any of the advantages of thepresent invention.

FIG. 1 depicts a schematic representation of an electrochemical cell;

FIG. 1a is a schematic representation of one example of a batteryemploying the electrochemical cells of the invention;

FIG. 2a is a representation of a powder X-ray diffraction pattern of aLi[Ti_(1.67)Mg_(0.33)]O₄ spinel product;

FIG. 2b is a representation of a powder X-ray diffraction pattern of aLi[Ti_(1.67)Li_(0.30)Mg_(0.03)]O₄ spinel product;

FIG. 2c is a representation of a powder X-ray diffraction pattern of aLi[Ti_(1.67)Li_(0.25)Al_(0.08)]O₄ spinel product;

FIG. 3a is a graphical representation of the voltage profiles for thefirst ten cycles of a Li/Li[Ti_(1.67)Mg_(0.33)]O₄ cell;

FIG. 3b is a graphical representation of the capacity versus cyclenumber for a Li/Li[Ti_(1.67)Mg_(0.33)]O₄ cell over the first nineteencycles;

FIG. 4a is a graphical representation of the voltage profiles for thefirst eight cycles of a Li/Li[Ti_(1.67)Li_(0.30)Mg_(0.03)]O₄ cell;

FIG. 4b is a graphical representation of the capacity versus cyclenumber for a Li/Li/Li[Ti_(1.67)Li_(0.30)Mg_(0.03)]O₄ cell over the firsttwenty two cycles. The performance of a standardLi/Li[Ti_(1.67)Li_(0.33)]O₄ cell is given for comparison.

FIG. 5a is a graphical representation of the voltage profiles for thefirst eight cycles of a Li/Li[Ti_(1.67)Li_(0.25)Al_(0.08)]O₄ cell;

FIG. 5b is a graphical representation of the capacity versus cyclenumber for a Li/Li[Ti_(1.67)Li_(0.30)Al_(0.03)]O₄ cell over the firsttwenty two cycles and a Li/Li[Ti_(1.67)Li_(0.25)Al_(0.08)]O₄ cell overthe first eighteen cycles. The performance of a standardLi/Li[Ti_(1.67)Li_(0.33)]O₄ cell is given for comparison.

FIG. 6 is a plot of the electronic conductivity (S cm⁻¹) versuscomposition (x) for various Li_(4−x)Mg_(x)Ti₅O₁₂ materials;

FIG. 7 is a plot of delivered electrode capacity versus discharge ratefor various Li/Li_(4−x)Mg_(x)Ti₅O₁₂ cells; and

FIG. 8 is a plot of the area specific impedance (ASI) (ohm cm²) forvarious Li/Li_(4−x)Mg_(x)Ti₅O₁₂ cells.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings and particularly to FIG. 1, there isdisclosed an electrochemical cell 10 having an anode 12 separated by anelectrolyte 14 and a cathode 16, all contained in an insulating housing18 with the anode separated from the cathode by the electrolyte andsuitable terminals (not shown) being provided in electrical contactrespectively with the anode 12 and the cathode 16. FIG. 1a shows aschematic representation of one sample of a battery in which two stringsof cells are in parallel and each string comprises three cells inseries. Binders and other materials normally associated with both theelectrolyte and the anode and the cathode are well known and are notdescribed herein, but are included as is understood by those of ordinaryskill in the art. The electrodes of the subject invention are based uponlithium titanium oxide spinel. Li₄Ti₅O₁₂ is a stoichiometric spinel thathas the spinel notation Li[Ti_(1.67)Li_(0.33)]O₄. The structure has theconventional spinel configuation A[B₂]X₄ where A refers to thetetrahedrally-coordinated cations on the crystallographic 8 a sites andB refers to the octahedrally-coordinated cations on the crystallographic16 d sites of the prototypic spinel space group Fd3m which has cubicsymmetry. The X anions, that form a cubic-close-packed array, arelocated at the 32 e positions of the space group. The general name forspinel compounds is derived from the mineral “spinel” Mg[Al₂]O₄.

It is now well known that lithium can be inserted into many spinelcompounds at room temperature according to the reaction:

Li+A _(8a) [B ₂]_(16d) X ₄- - ->(LiA)_(16c) [B ₂]_(16d) X ₄

During this reaction, the tetrahedral A cations are displaced intoneighboring octahedral sites to generate a rock salt structure(LiA)_(16c)[B₂]_(16d)X₄ in which all the octahedral sites are occupiedby the Li, A and B cations. During the lithiation process, the [B₂]X₄spinel framework remains intact. The interstitial space of the [B₂]X₄spinel framework provides a three-dimensional network of the 8 atetrahedra and 16 c octahedra through which the lithium ions are able todiffuse. Thus, when the A cations are lithium ions, the lithium ions areable to diffuse in an unrestricted manner through the interstitialspace, thus allowing rapid lithium-ion transport.Li[Ti_(1.67)Li_(0.33)]O₄ is an example of such a spinel, a majorlimitation being its insulating character because all the titanium ionsare tetravalent.

In a first embodiment of the invention, there is provided a family ofmodified spinel compounds in which the lithium ions on the octahedral Bsites of Li[Ti_(1.67)Li_(0.33)]O₄ are partially replaced by eithermagnesium and/or aluminum. This family of spinel compounds can berepresented by the general formula Li[Ti_(1.67)Li_(0.33−y)M_(y)]O₄, for0<y≦0.33, where M=Mg and/or Al. For the case where M=Mg, the one endmember of the solid solution series, i.e., without any M cations, isLi[Ti_(1.67)Li_(0.33)]O₄(y=0), described above in which all the titaniumions are tetravalent; Li[Ti_(1.67)Li_(0.33)]O₄ is thus an insulator. Theother end member is Li[Ti_(1.67)Mg_(0.33)]O₄(y=0.33), in which thelithium ions on the octahedral 16 d sites have been completelysubstituted by magnesium. In Li[Ti_(1.67)Mg_(0.33)]O₄, the titanium ionsare of mixed Ti⁴⁺/Ti³⁺ valence, the average oxidation state being 3.80,i.e., the compound is a mixed-valent spinel with enhanced electronicconductivity compared to Li[Ti_(1.67)Li_(0.33)]O₄. InLi[Ti_(1.67)Mg_(0.33)]O₄ the magnesium ions occupy the B sites of thespinel structure and, therefore, leave the interstitial space of 8 atetrahedra and 16 c octahedra available for the unrestricted diffusionof lithium ions. For this electrode composition, the electrochemicalreaction is:

Li+Li[Ti_(1.67)Mg_(0.33)]O₄→Li₂[Ti_(1.67)Mg_(0.33)]O₄

In the discharged (rock salt) product, Li₂[Ti_(1.67)Mg_(0.33)]O₄, thatrepresents a fully charged negative electrode, the titanium ions arereduced to an average oxidation state of 3.19, leaving the spinelstructure with mixed valence (Ti⁴⁺ and Ti³⁺) and hence higher electronicconductivity. Thus, the electrode maintains mixed valent Ti⁴⁺/Ti³⁺character throughout charge and discharge with enhanced electricalconductivity over the parent compound Li₄Ti₅O₁₂. It is also possible tofabricate Li[Ti_(1.67)Li_(0.33−y)Mg_(y)]O₄ spinel compounds withintermediate values of y, such as Li[Ti_(1.67)Li_(0.30)Mg_(0.03)]O₄thereby tailoring the amount of Ti⁴⁺ and Ti³⁺ ions in the startingspinel electrode structure and in the final lithiated spinel (rock salt)structure. Mg substitution in the spinel structure does notsignificantly alter the capacity of the electrode. For example, thetheoretical capacity of Li[Ti_(1.67)Mg_(0.33)]O₄ (169 mAh/g) is onlyslightly smaller than that of Li[Ti_(1.67)Li_(0.33)]O₄ (175 mAh/g). Aparticular advantage of the Mg-substituted spinel materials is that forthe lower concentrations of Mg substitution, although prepared underinert atmospheric conditions, the materials are stable in air at roomtemperature.

When aluminum is used as the M cation, the cation-substituted electrodeis represented by Li[Ti_(1.67)Li_(0.33−y)Al_(y)]O₄ (0<y≦0.33). In thefully-substituted compound (y=0.33), the average oxidation state of thetitanium ions is 3.59, and in the fully lithiated compoundLi₂[Ti_(1.67)Al_(0.33)]O₄ which represents a fully charged negativeelectrode, the average oxidation state is 3.0. Because it is desirableto keep the average oxidation state of the titanium ions between 3.0 and4.0 during all states of charge and discharge, it is preferable to keepthe value of y below 0.33 when Al is used as the only M cation. Like Mgsubstitution, Al substitution does not significantly alter the capacityof the spinel electrode. For example, the theoretical capacity ofLi[Ti_(1.67)Al_(0.33)]O₄ (168 mAh/g) is only slightly smaller than thatof Li[Ti_(1.67)Li_(0.33)]O₄ (175 mAh/g). The invention extends toinclude compositions Li[Ti_(1.67)Li_(0.33−y)M_(y)]O₄ (0<y≦0.33), inwhich M can be Mg and Al, for example,Li[Ti_(1.67)Mg_(0.167)Al_(0.167)]O₄.

In a second embodiment of the invention, there is provided a family ofspinel compounds Li[Ti_(1.67)Li_(0.33−y)M_(y-z)M′_(z)]O₄ that can bederived from Li[Ti_(1.67)Li_(0.33−y)M_(y)]O₄ by partial substitution ofthe M cations by M′ cations where 0<y≦0.33, z<y, M is Mg²⁺ and/or Al³⁺,and M′ is one or more suitable monovalent, divalent, trivalent andtetravalent metal cations. By suitable, we mean that the M′ cations areselected preferably from the first row of transition metal elements, Sc,Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, and more preferably from Ti⁴⁺,Co³⁺, Co²⁺, Ni³⁺ and Ni²⁺ such that the fully charged and fullydischarged (negative) spinel electrodes contain a mixed-valent Ti⁴⁺/Ti³⁺couple. For example, when M=Mg²⁺, and M′=Ti⁴⁺, x=0.33, and y=0.17, theelectrode would have the composition Li[Ti_(1.67)Mg_(0.17)Ti_(0.17)]O₄,or alternatively, Li[Ti_(1.84)Mg_(0.17)]O₄. In this example, the averageoxidation state of the titanium ions in the discharged negativeelectrode is 3.62, whereas in the lithiated fully-charged negativeelectrode Li₂[Ti_(1.84)Mg_(0.17)]O₄, the average oxidation state of thetitanium ions is 3.08, in compliance with the need for mixed Ti⁴⁺/Ti³⁺to ensure enhanced electronic conductivity at all states of charge anddischarge. Thus, it can be understood from the principles of thisinvention that a variety of monovalent, divalent, trivalent ortetravalent M′ metal cations can be used with the M cations to ensureTi⁴⁺/Ti³⁺ mixed valence thus increasing the electrical conductivity ofthe lithium-titanium-oxide spinel electrode. Furthermore, it can beunderstood that the M′ cations are selected preferably from those ionsthat have ionic radii comparable to the Li⁺, Ti⁴⁺ and Ti³⁺ ions withinthe [Ti_(1.67)Li_(0.33−y)M_(y)]O₄ spinel framework, preferably within0.15 Å.

In practice, it is difficult to prepare lithium spinel compounds withthe precise stoichiometric a Li[B₂]O₄ formula, where B is a metalcation; these spinel compounds often contain defects or vacancies, andsometimes a small degree of spinel inversion. For example, it ispossible that a small amount of the substituted metal cation M, such asmagnesium or aluminum may be found on the tetrahedral 8 a sites as wellas the octahedral 16 d sites of the spinel structure without causing asignificant change to the electrochemical properties of the spinelelectrode. Indeed, a small amount of dopant cation on the tetrahedralsites may serve to stabilize the interstitial space of the spinelstructure to lithium insertion/extraction. This invention embodies theseslight changes in composition and site occupancies of the spinelstructure. The principles of this invention are demonstrated withrespect to the compounds Li[Ti_(1.67)Mg_(0.33)]O₄ (alternatively,Li₃MgTi₅O₁₂), Li[Ti_(1.67)Li_(0.30)Mg_(0.03)]O₄ (alternatively,Li_(3.9)Mg_(0.1)Ti₅O₁₂), and Li[Ti_(1.67)Li_(0.25)Al_(0.08)]O₄(altematively, Li_(3.75)Al_(0.25)Ti₅O₁₂)

Experimental EXAMPLE

Li[Ti_(1.67)Li_(0.33−x)Mg_(x)]O₁₂ materials were prepaed typically byreacting LiOHH₂O, TiO₂ (anatase) and Mg(OH)₂ or Mg(NO₃)₂ in therequired stoichiometric amounts. The materials were first intimatelymixed and calcined at 1000° C. for 6 hours under a helium atmospherecontaining 3% hydrogen. Unlike Li[Ti_(1.67)Li_(0.33)]O₄(Li₄Ti₅O₁₂),which is white and an insulator, the Li[Ti_(1.67)Li_(0.33−y)Mg_(y)]O₄products were blue to black in color, indicating that the spinelstructure had mixed-valent Ti⁴⁺/Ti³⁺ character, with electronicconductivity. The powder X-ray diffraction pattern of theLi[Ti_(1.67)Mg_(0.33)]O₄ (x=0.33) product is shown in FIG. 2a, and thepattern of Li[Ti_(1.67)Li_(0.30)Mg_(0.03)]O₄ (y=0.03) is shown on FIG.2b. The patterns are characteristic of single-phase spinel compounds.The X-ray diffraction patterns of products with y>0.33 generally showedseveral phases, typically a spinel phase that resembledLi[Ti_(1.67)Mg_(0.33)]O₄, and in addition, phases resembling Li₂MgTi₃O₈and MgTi₂O₅.

EXAMPLE 2

Li[Ti_(1.67)Li_(0.33−y)Al_(y)]O₄ materials were prepared typically byreacting LiOH H₂O, TiO₂ (anatase) and Al(NO₃)₃ in the requiredstoichiometric amounts. The materials were first intimately mixed andcalcined at 1000° C. for 6 hours under a helium atmosphere containing 3%hydrogen. Unlike Li[Ti_(1.67)Li_(0.33)]O₄(Li₄Ti₅O₁₂), which is white andan insulator, the Li[Ti_(1.67)Li_(0.33−y)Al_(y)]O₁₂ products wereblue/black in color, indicating that the spinel structure hadmixed-valent Ti⁴⁺/Ti³⁺ character, with electronic conductivity. Thepowder X-ray diffraction pattern of the single-phaseLi[Ti_(1.67)Li_(0.25)Al_(0.08)]O₄ (y=0.33) product is shown in FIG. 2c.Products with y>0.1 tended to consist of several phases, typically acubic spinel phase that resembled Li[Ti_(1.67)Al_(0.33)]O₄, and inaddition, a TiO₂ (rutile) phase and a second spinel phase LiAl₅O₈. It isbelieved that by improving processing techniques, it will be possible toobtain single-phase Li[Ti_(1.67)Li_(0.33−x)Al_(x)]O₄ products for therange 0<y≦0.33.

EXAMPLE 3

Li[Ti_(1.67)Li_(0.33−y)M_(y)]O₄ materials (M=Mg, Al) were evaluated inlithium coin cells (size 1225) with the configuration: Li/1 M LiPF₆ inethylene carbonate:dimethyl carbonate(50:50)/Li[Ti_(1.67)Li_(0.33−y)M_(y)]O₄. Metallic lithium was used asthe negative electrode for the evaluations. The positive electrodeconsisted of a blend of 81% active Li[Ti_(1.67)Li_(0.33−y)M_(y)]O₄spinel material, 10% binder (Kynar 2801) and 9% carbon (XC-72),laminated onto an aluminum foil substrate. Cells were charged anddischarged at a constant current rate of between 0.1 mA/cm² and 0.25mA/cm² between voltage limits of 3.0 and 0.5 V. The voltage profiles ofthe first twenty cycles for Li/Li[Ti_(1.67)Mg_(0.33)]O₄,Li/Li[Ti_(1.67)Li_(0.30)Mg_(0.03)]O₄ andLi/Li[Ti_(1.67)Li_(0.25)Al_(0.08)]O₄ cells are shown in FIGS. 3a, 4 aand 5 a, respectively. These figures show that most of the capacity ofthese cells is delivered between 1.5 and 1.3 V. End-of-charge andend-of-discharge is indicated by a rapid increase and decrease in cellvoltage respectively. The good retention of capacity of the cells oncycling is shown in FIGS. 3b, 4 b, and 5 b, respectively, in which thecapacity of the spinel electrodes decreases only marginally over thefirst twenty cycles. Thus the data in FIGS. 3a and b, 4 a and b, and 5 aand b, indicate the excellent potential that substitutedlithium-titanium-oxide spinel materials have as electrodes forrechargeable lithium batteries.

EXAMPLE 4

The electronic conductivity of Li[Ti_(1.67)Li_(0.33−y)M_(y)]O₄ materialswas determined by a standard four-point probe technique using acompacted disc of Li[Ti_(1.67)Li_(0.33−y)M_(y)]O₄ between two indiumelectrodes. A plot of the conductivity versus composition forLi[Ti_(1.67)Li_(0.33−y)Mg_(y)]O₄ samples (alternativelyLi_(4−x)Mg_(x)Ti₅O₁₂) is shown in FIG. 6. The plot shows that theelectronic conductivity of the spinel samples increases significantlywith Mg substitution to reach a maximum value of 0.1 S cm⁻¹ at acomposition Li[Ti_(1.67)Mg_(0.33)]O₄; this composition coincides withthe fall replacement of Li by Mg on the octahedral 16 d sites of thespinel structure. In the aluminum-doped samples,Li[Ti_(1.67)Li_(0.33−y)Al_(y)]O₄, a significant improvement in theelectronic conductivity was also observed in accordance with theprinciples of this invention. For example, the conductivity ofsingle-phase Li[Ti_(1.67)Li_(0.25)Al_(0.08)]O₄ was 8×10⁻⁵ S cm⁻¹, i.e.,many orders of magnitude greater than that observed in a standardLi[Ti_(1.67)Li_(0.33)]O₄ (Li₄Ti₅O₁₂) sample. BecauseLi[Ti_(1.67)Li_(0.33)]O₄ is such an excellent electronic insulator, itsexact electronic conductivity could not be determined because it wasbelow the resolution of the measuring equipment (typically less than10⁻¹² S cm⁻¹).

EXAMPLE 5

The improvement in the electronic conductivity of the electrodes of theinvention was also evaluated by monitoring the current rate capabilityand area specific impedance (ASI) of Li/Li[Ti_(1.67)Li_(0.33−y)M_(y)]O₄cells. The current rate capability was evaluated by determining thecapacity that could be obtained from the electrodes at various currentrates. For example, FIG. 7 demonstrates thatLi[Ti_(1.67)Li_(0.33−y)Mg_(y)]O₄ electrodes with y=0.08 and 0.167(alternatively Li_(4−x)Mg_(x)Ti₅O₁₂ with x=0.25 and 0.5) providesignificantly greater capacities when discharged at higher current ratesthan standard Li[Ti_(1.67)Li_(0.33)]O₄ (Li₄Ti₅O₁₂) electrodes. FIG. 8shows the area specific impedance of the cells, calculated from therelaxation voltage (delta V) after a 30-second interrupt and thedischarge current (0.25 mA/cm²). FIG. 8 clearly shows that the cellimpedance of Li/Li[Ti_(1.67)Li_(0.33−y)Mg_(y)]O₄ cells with y=0.08 andy=0.167 (alternatively Li_(4−x)Mg_(x)Ti₅O₁₂ with x=0.25 and 0.5) issignificantly lower than that of cells with standardLi[Ti_(1.67)Li_(0.33)]O₄ (Li₄Ti₅O₁₂) spinel electrodes.

While there has been disclosed what is considered to be the preferredembodiment of the present invention, it is understood that variouschanges in the details may be made without departing from the spirit, orsacrificing any of the advantages of the present invention.

We claim:
 1. A negative electrode for a non-aqueous electrochemicallithium cell having a spinel-type structure with the general formulaLi[Ti_(1.67)Li_(0.33−y)M_(y)]O₄, for 0<y≦0.33, where M=Mg and/or Al. 2.A negative electrode of claim 1, wherein the M cations are partiallyreplaced by one or more suitable divalent, trivalent or tetravalentmetal M′ cations to provide an electrodeLi[Ti_(1.67)Li_(0.33−y)M_(y-z)M′_(z)]O₄ in which z<y.
 3. A negativeelectrode of claim 2, wherein the M′ cations are selected from the firstrow of transition metal elements.
 4. A negative electrode of claim 3,wherein the M′ cations are selected from Ti⁴⁺, Co³⁺, Ni³⁺, Co²⁺ andNi²⁺.
 5. A non-aqueous lithium electrochemical cell comprising anegative electrode, an electrolyte and a positive electrode, thenegative electrode consisting of a spinel-type structure with thegeneral formula Li[Ti_(1.67)Li_(0.33−y)M_(y)]O₄, for 0<y≦0.33, whereM=Mg and/or Al.
 6. A non-aqueous lithium electrochemical cell of claim 5comprising a negative electrode, an electrolyte and a positiveelectrode, the negative electrode consisting of aLi[Ti_(1.67)Li_(0.33−y)M_(y)]O₄ spinel-type structure in which the Mcations are partially replaced by one or more suitable monovalent,divalent, trivalent or tetravalent metal M′ cations to provide anelectrode Li[Ti_(1.67)Li_(0.33−y)M_(y-z)M′_(z)]O₄ in which z<y, whereinM′ cations are selected from one or more of suitable divalent, trivalentor tetravalent metal M′ cations to provide an electrodeLi[Ti_(1.67)Li_(0.33−y)M_(y-z)M′_(z)]O₄ in which z<y.
 7. A non-aqueouslithium electrochemical cell of claim 6 wherein the M′ cations areselected from the first row of transition metal elements.
 8. Anon-aqueous lithium battery comprising an plurality of cells,electrically connected, each cell comprising a negative electrode, anelectrolyte and a positive electrode, the negative electrode consistingof a spinel-type structure with the general formulaLi[Ti_(1.67)Li_(0.33−y)M_(y)]O₄, for 0<y≦0.33, where M=Mg and/or Al. 9.A non-aqueous lithium battery of claim 8 wherein the M cations of thenegative electrode are partially replaced by one or more suitablemonovalent, divalent, trivalent or tetravalent metal M′ cations toprovide an electrode Li[Ti_(1.67)Li_(0.33−y)M_(y-z)M′_(z)]O₄ in whichz<y, and wherein M′ cations are selected from one or more of suitabledivalent, trivalent or tetravalent metal M′ cations to provide anelectrode Li[Ti_(1.67)Li_(0.33−y)M_(y-z)M′_(z)]O₄ in which z<y.
 10. Aspinel-type material with the general formulaLi[Ti_(1.67)Li_(0.33−y)M_(y)]O₄, for 0<y≦0.33, where M=Mg and/or Al,wherein the M cations are partially replaced by one or more suitabledivalent, trivalent or tetravalent metal M′ cations to provide anelectrode Li[Ti_(1.67)Li_(0.33−y)M_(y-z)M′_(z)]O₄ in which z<y.
 11. Thespinel-type material of claim 10, wherein the M′ cations are selectedfrom the first row of transition metal elements.
 12. The spinel-typematerial of claim 11, wherein the M′ cations are selected from Ti⁴⁺,Co³⁺, Ni³⁺, Co²⁺ and Ni²⁺.
 13. A negative electrode for a non-aqueouselectrochemical lithium cell having a spinel-type structure with thegeneral formula Li[Ti_(1.67)Li_(0.33−y)M_(y)]O₄, for 0<y≦0.33, whereM=Mg and/or Al, wherein the M cations are partially replaced by one ormore suitable divalent, trivalent or tetravalent metal M′ cations toprovide an electrode Li[Ti_(1.67)Li_(0.33−y)M_(y-z)M′_(z)]O₄ in whichz<y, and wherein the M′ cations are selected from the first row oftransition metal elements.
 14. A spinel-type material with the generalformula Li[Ti_(1.67)Li_(0.33−y)M_(y)]O₄, for 0<y≦0.33, where M=Mg and/orAl, wherein the M cations are partially replaced by one or more suitabledivalent, trivalent or tetravalent metal M′ cations to provide anelectrode Li[Ti_(1.67)Li_(0.33−y)M_(y-z)M′_(z)]O₄ in which z<y, whereinthe M′ cations are selected from Ti⁴⁺, Co³⁺, Ni³⁺, Co²⁺ and Ni²⁺.